Details:
Summary:
Two-component inks are used in the manufacture of coloured contact lens by pad printing. As the ink is reacting through out the print production run, the physical characteristics are continually changing. This has a significant impact on the ink transfer. This paper summarises the results of an extensive experimental investigation into the rheological behaviour of two-component inks and their main components such as binders and slurries. The viscosity of low and high viscosity binders as well as six coloured slurries falls as the shear rate increases, indicating shear thinning behaviour. The test results suggested that binders and slurries are strongly dependent on shear rate, frequency, and temperature. The results from thixotropic loop tests confirmed different degrees of thixotropy for binders and coloured slurries and highlighted a maximum for yellow slurry. At all times the high viscosity binder shows a more elastic behaviour than the low viscosity binder. The elastic component is smaller than the viscous component for both binders. The fact that the low viscosity binder is easier to print is reflected by the lower value of the loss modulus. The results from the flow curve tests confirmed the effect of solvent evaporation and polymerization on ink viscosity over period of four hours. The viscosity of the ink increased with time suggesting polymerisation occurred within the ink. The effects of these physical changes are discussed in relation to the printed properties of the lens and issues of quality assurance.